Highly crystalline silver powder and method for producing the same

ABSTRACT

An object of the present invention is to provide highly crystalline silver powder which is characterized in fine particles, showing high dispersibility, it&#39;s particle size distribution is not excessively sharp but relatively broad and crystallites are large; and a method for producing the same. In order to achieve the object, a method for producing highly crystalline silver powder is characterized in that mixing a first aqueous solution and a second aqueous solution, wherein the first aqueous solution contains silver nitrate, a dispersing agent and nitric acid, and the second solution contains ascorbic acid. For dispersing agent, polyvinylpyrrolidone or gelatin is preferred. Highly crystalline silver powder produced by the above-described method is preferred to be a crystallite diameter of 300 Å or more, an average particle diameter D 50  in the range from 0.5 μm to 10 μm, and a thermal shrinkage rate for the length direction after heating at 700° C. in the range from −3% to 3%. For ratio D 90 /D 10  of the silver powder is preferred to be in the range from 2.1 to 5.0.

TECHNICAL FIELD

The present invention relates to highly crystalline silver powder and amethod for producing the same, and more specifically, to highlycrystalline silver powder preferable for production of a conductivepaste that can significantly reduce the size of the electrode or circuitof, for example, a chip devices, plasma display panel and the like withhigh density, high accuracy and high reliability. In particular it canenable to form a minute wiring or a thin and flat coating film with highdensity, high accuracy and high reliability. It is because the silverpowder is composed of fine particles, has high dispersibility, particlesize distribution is not excessively sharp but is relatively broad, andcrystallites are large. So when it is used as the material for theconductive paste, the dispersibility of the silver powder and thefilling properties of the paste are excellent, and then the size of theelectrode or circuit formed from the thick silver film can be reduced,and the thick silver film obtained from the conductive paste excels inthermal shrinkage resistance, and has low specific resistance(resistivity).

BACKGROUND ART

Heretofore, as a method for forming an electrode or circuit of anelectronic part or the like, there has been known a method to form acircuit by printing a conductive paste wherein silver powder, which is aconductive material, is dispersed in a paste, on a substrate, and bakingor curing the paste to form a thick silver film. However in recentyears, due to the improvement in functions of electronic equipment, socalled down sizing and wiring density increase of electronic deviceshave been demanded; consequently, it has been desired that silverpowder, which is the material for the conductive paste, excels infilling properties and dispersibility despite of fine particles whenused in the conductive paste. In the present invention, dispersibilitymeans the difficulty of aggregating the primary particles of the silverpowder with each other, unless otherwise specified such as thedispersibility of silver powder in a paste. For example, the state ofhigh dispersibility means the state wherein there is little or noproportion of primary particles are aggregated with each other; and thestate of low dispersibility means the state wherein there is muchproportion or all of aggregated primary particles are aggregated witheach other.

A substrate on which the above-described conductive paste is printed isnormally used in a part of a ceramic substrate where heat generation islarge, such as the IC package. However, when the conductive paste isprinted on the ceramic substrate, since the thermal shrinkage of theceramic substrate is generally different from the thermal shrinkage ofthe thick silver film formed from the conductive paste, there ispossibility wherein the thick silver film is separated from the ceramicsubstrate, or the substrate itself is deformed. Therefore, it ispreferable that the rate of the thermal shrinkage of the ceramicsubstrate is as close to the rate of the thermal shrinkage of the thicksilver film formed from the conductive paste.

As a cause of the thermal shrinkage of the above-described thick silverfilm in such baking process, it is considered that the silver powder inthe conductive paste causes sintering during baking. Specifically, it isconsidered that the silver powder is a polycrystalline constructioncomposed of fine crystallites, and when the conductive paste containingsilver powder is baked for the formation of the thick silver film, thefine crystallites in the silver powder are sintered, and dimensionchange between before and after formation of the sintered thick silverfilm cause thermal shrinkage. Therefore, in order to obtain a conductivepaste containing silver powder with less thermal shrinkage, it isdesirable that the crystallites in the silver powder are as large aspossible so as to minimize the sintering of the crystallites.

In recent years, the improvement of the high-frequency signalcharacteristics of circuits and the improvement of dimensional stabilityof substrates before and after sintering are demanded, and for this, asa substrate on which the thick silver film is formed, an LTCC (lowtemperature co-baked ceramic) substrate has been used substituting theabove-described ordinary ceramic substrates. Furthermore, since the LTCCsubstrate is obtained by sintering a green sheet of the LTCC substratewith a conductive paste containing a low-resistance conductor such assilver powder together, compared with the technique to form the circuitof a thick silver film by printing a conductive paste using theabove-described ordinary ceramic substrate, the number of sinteringsteps is less, the film thickness of the ceramic dielectric can beeasily controlled, the conductor resistance of the circuit formed fromthe conductive paste is lowered, and the coplanarity of the substratecan be easily improved. However, since the LTCC is much excellent indimensional stability, silver powder, which is the material of theconductive paste used in it is strongly required to have less thermalshrinkage, and therefore, it is strongly desired that the crystallitesin the silver powder be large.

If the crystallites in the silver powder are large as described above,the content of impurities in the silver powder is generally lowered, andthereby the specific resistance of the circuit formed from the thicksilver film is easily lowered; therefore, this is also preferable in theaspect that the conductive paste containing silver powder can be usednot only in the circuit formed by baking as described above, but also inthe circuit formed without baking.

As described above, it is desired for silver powder used in theconductive paste, that the silver powder which is composed of fineparticles, having high dispersibility, whose particle size distributionis not excessively sharp but relatively broad, and the crystallites arelarge.

Whereas in Patent Document 1 (Japanese Patent Laid-Open No. 2000-1706),a method for producing highly crystalline silver powder wherein anaqueous solution of silver nitrate and a solution prepared by dissolvingacrylic acid monomer in an aqueous solution of L-ascorbic acid are mixedand allowed to react simultaneously. According to the above-describedmethod, highly crystalline silver powder whose crystallite size is 400 Åor more, and the range of the particle diameters is as narrow as 2 to 4μm can be obtained.

Patent Document 1: Japanese Patent Laid-Open No. 2000-1706 (page 1)

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, although the silver powder described in Patent Document 1 iscomposed of fine particles with large crystallites, it is difficult thatthe thermal shrinkage at a high temperature of for example about 700° C.is sufficiently reduced. The silver powder has a large thermal shrinkageat a high temperature even if the crystallites are sufficiently large,and the reason is estimated to be caused by that the range of theparticle diameters of the silver powder is between 2 and 4 μm, and sincethe particle size distribution is excessively sharp, gaps are formedbetween the particles of the silver powder, and the filling of thesilver powder is lowered. Therefore, when the silver powder was used toprepare a conductive paste to form a thick silver film or to form acircuit using an LTCC substrate, the dimension change between before andafter forming the circuit was enlarged causing a problem of warpage inan ordinary ceramic substrate or LTCC substrate, particularly the LTCCsubstrate.

Therefore, it is an object of the present invention to provide highlycrystalline silver powder composed of fine particles, having highdispersibility, whose particle size distribution is not excessivelysharp but relatively broad, and the crystallites are large; and a methodfor producing the same.

Means for Solving the Problems

Under such a situation, the present inventors carried out keen studies,and found to complete the present invention. It is that if silver powderis produced using a method to mix a first aqueous solution containingsilver nitrate, a dispersing agent and nitric acid, and a second aqueoussolution containing ascorbic acid, highly crystalline silver powdercomposed of fine particles, having high dispersibility, whose particlesize distribution is not excessively sharp but relatively broad, and thecrystallites are large. And it can make the thick silver film obtainedfrom the conductive paste excel in thermal shrinkage resistance, can beobtained.

Specifically, the present invention provides a method for producinghighly crystalline silver powder which is characterized in that mixing afirst aqueous solution and a second aqueous solution, wherein the firstaqueous solution contains silver nitrate, a dispersing agent and nitricacid, and the second solution contains ascorbic acid.

The present invention also provides the method for producing highlycrystalline silver powder characterized in that the dispersing agent ispolyvinylpyrrolidone.

The present invention function provides the method for producing highlycrystalline silver powder characterized in that the dispersing agent isa gelatin.

The present invention still further provides the method for producinghighly crystalline silver powder characterized in that the first aqueoussolution when it contains 100 parts by weight of silver nitrate, itfurther contains 5 parts by weight to 60 parts by weight ofpolyvinylpyrrolidone and 35 parts by weight to 70 parts by weight ofnitric acid.

The present invention also provides the method for producing highlycrystalline silver powder characterized in that the first aqueoussolution when it contains 100 parts by weight of silver nitrate, itfurther contains 0.5 parts by weight to 10 parts by weight of gelatinand 35 parts by weight to 70 parts by weight of nitric acid.

The present invention further provides the method for producing highlycrystalline silver powder characterized in that the first aqueoussolution contains 100 parts by weight of silver nitrate, ascorbic acidcontained in the second aqueous solution to be mixed with the firstaqueous solution is 30 parts by weight to 90 parts by weight.

The present invention still further provides the method for producinghighly crystalline silver powder characterized in that the secondaqueous solution contains 100 parts by weight of ascorbic acid, nitricacid contained in the first aqueous solution to be mixed with the secondaqueous solution is 40 parts by weight to 150 parts by weight.

The present invention also provides highly crystalline silver powdercharacterized in that the powder is produced by the method for producinghighly crystalline silver powder.

The highly crystalline silver powder produced by the method forproducing highly crystalline silver powder is characterized in that thepowder has a crystallite diameter of the powder is 300 Å or more.

The highly crystalline silver powder is characterized in that an averageparticle diameter D₅₀ of the powder is in the range from 0.5 μm to 10μm. (where D₅₀ is a median diameter (μm) calculated as 50% of volumecumulative distributions examined by a laser diffraction scatteringparticle size distribution measuring method).

The highly crystalline silver powder is characterized in that a thermalshrinkage rate of the powder after heating at 700° C. is in the rangefrom −3% to 3%.

The highly crystalline silver powder is characterized in that a ratioD₉₀/D₁₀ of the powder is in the range from 2.1 to 5.0 (where D₁₀ isdiameter (μm) at 10% of volume cumulative distributions and D₉₀ isdiameter (μm) at 90% of volume cumulative distributions examined by alaser diffraction scattering particle size distribution measuringmethod, respectively).

The highly crystalline silver powder is characterized in that acrystallite diameter is 300 Å or more, an average particle diameter D₅₀is in the range from 0.5 μm to 10 μm, and a thermal shrinkage ratioafter heating at 700° C. in the length direction is in the range from−3% to 3%.

The highly crystalline silver powder is characterized in that a ratioD₉₀/D₁₀ of the powder is in the range from 2.1 to 5.0 (where D₁₀ isdiameter (μm) at 10% of volume cumulative distributions and D₉₀ isdiameter (μm) at 90% by volume of cumulative distributions examined by alaser diffraction scattering particle size distribution measuringmethod, respectively).

ADVANTAGE OF THE INVENTION

Since the highly crystalline silver powder according to the presentinvention is of fine particles, has high dispersibility, and whoseparticle size distribution is not excessively sharp and relativelybroad, and crystallites are large. And when it is used as a material fora conductive paste, the dispersibility of the silver powder to theconductive paste and the filling property of the conductive paste withthe silver powder in can be excellent; an electrode, circuit and thelike can be finer; the thick silver film obtained from the conductivepaste can be excellent in thermal shrinkage resistance; and theresistivity thereof can be lowered. The method for producing the highlycrystalline silver powder according to the present invention canefficiently produce the above-described highly crystalline silver powderaccording to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION (Highly Crystalline SilverPowder According to the Present Invention)

The highly crystalline silver powder according to the present inventionis substantially granular powder. The average particle diameter D₅₀ ofthe highly crystalline silver powder according to the present inventionis 0.5 μm to 10 μm, preferably 1 μm to 5 μm. It is preferable that theaverage particle diameter D₅₀ is within the above-described rangebecause when the silver powder is used in a conductive paste, thefilling of the silver powder in the conductive paste is excellent, andthe circuit or the like formed from the thick silver film can be madefiner. On the other hand, the average particle diameter D₅₀ less than0.5 μm is not preferable because the collection of the silver powderbecomes difficult; and the average particle diameter D₅₀ exceeding 10 μmis not preferable because the silver powder is often aggregated. Here,the average particle diameter D₅₀ means the volume average particlediameter measured by a laser diffraction scattering method, that is, theparticle diameter at 50% cumulative distribution.

The highly crystalline silver powder according to the present inventionhas a crystallite diameter of 300 Å or more, preferably 350 Å to 600 Å.It is preferable that the crystallite diameter is within theabove-described range because when the conductive paste containing thesilver powder is applied onto a ceramic substrate, and baked to form acircuit or the like composed of a thick silver film, the thermalshrinkage of the thick silver film between before and after bakingbecomes close to the thermal shrinkage of the ceramic substrate, and theeffect to suppress the separation of the thick silver film from theceramic substrate, or the deformation of the ceramic substrate due tothe dimension change of the thick silver film is large.

On the other hand, it is not preferable that the crystallite diameter isless than 300 Å, because when the conductive paste is applied onto aceramic substrate, and baked to form a circuit or the like composed of athick silver film, the shrinkage of the thick silver film between beforeand after baking becomes larger than the shrinkage of the ceramicsubstrate, and the thick silver film is easily separated from theceramic substrate or the ceramic substrate is easily deformed due to thedimension change of the thick silver film. Here, the crystallitediameter means the average crystallite diameter obtained from thehalf-value width of the diffraction angle of each crystal face obtainedby conducting X-ray diffraction for silver powder sample.

The D₉₀/D₁₀ of the highly crystalline silver powder according to thepresent invention is normally 2.1 to 5.0, preferably 2.5 to 4.7. In thepresent invention, D₁₀ is diameter (μm) at 10% by volume cumulativedistributions, and D₉₀ is diameter (μm) at 90% by volume cumulativedistributions examined by a laser diffraction scattering particle sizedistribution measuring method, respectively. The D₉₀/D₁₀ is an indicatorto indicate fluctuation, and a large D₉₀/D₁₀ indicates that thefluctuation of particle size distribution is large. It is preferablethat D₉₀/D₁₀ is within the above-described range, because the particlesize distribution of the silver powder is not excessively sharp, but isrelatively broad, and when a circuit is formed with a conductive pasteusing the silver powder, the filling properties of the silver powder andthe thermal shrinkage resistance of the circuit becomes excellent,specifically the dimension change of the circuit between before andafter baking is easily reduced.

On the other hand, it is not preferable that D₉₀/D₁₀ is less than 2.1,because the particle size distribution becomes excessively sharp, andwhen a circuit is formed with a conductive paste using the silverpowder, the filling properties of the silver powder easily becomesinferior, specifically, the dimension change of the circuit betweenbefore and after baking easily becomes large. It is also not preferablethat D₉₀/D₁₀ exceeds 5.0, because the particle size distribution of thesilver powder becomes excessively broad to make silver powder fillingproperties inferior when forming a circuit using a conductive pasteusing the silver powder, and thus the thermal shrinkage resistance ofthe circuit is easily deteriorated, specifically, the dimension changeof the circuit between before and after baking easily becomes large.

The thermal shrinkage of the highly crystalline silver powder accordingto the present invention at 700° C. in the length direction is normallyin the range from −3% to 3%, preferably in the range from −2% to 2%. Inthe present invention, the thermal shrinkage at 700° C. in the lengthdirection means the thermal shrinkage of a pellet in the lengthdirection measured using thermomechanical analysis (TMA) in a sample inwhich silver powder is formed into a pellet.

In the highly crystalline silver powder according to the presentinvention, the resistivity of the silver coating film baked at arelatively low temperature, for example 300° C., is low. Specifically,even if the highly crystalline silver powder is sintered at a lowtemperature, the resistivity of the sintered article can be easilyreduced. The reason why the resistivity of the silver coating film thusbaked at 300° C. is estimated to be because the movement of electrons inthe silver powder is smoothened due to large crystallite diameter.

The specific surface area of the highly crystalline silver powderaccording to the present invention is normally 0.10 m²/g to 1.0 m²/g,preferably 0.20 m²/g to 0.90 m²/g. The specific surface area of lessthan 0.10 m²/g is not preferable because the formation of the finerelectrode or circuit composed of a thick silver film tends to bedifficult. The specific surface area exceeding 1.0 m²/g is also notpreferable because the formation of paste of silver powder tends to bedifficult. The specific surface area in the present invention is the BETspecific surface area.

The tap density of the highly crystalline silver powder according to thepresent invention is normally 3.8 g/cm³ or more, preferably 4.0 cm³ to6.0 cm³. The tap density within the above-described range is preferablebecause the filling of the silver powder in the paste of the highlycrystalline silver powder is favorable in the preparation of theconductive paste to facilitate the preparation of the conductive paste,and on the coating film formation of the conductive paste, adequate gapsare formed between the particles of the highly crystalline silver powderto facilitate binder removal from the coating film on baking the coatingfilm and to improve the density of the baked film, and as a result, theresistance of the thick silver film can be easily lowered. The highlycrystalline silver powder according to the present invention can beproduced by the following method.

(Method for Producing the Highly Crystalline Silver Powder According tothe Present Invention)

The method for producing the highly crystalline silver powder accordingto the present invention is to mix a first aqueous solution containingsilver nitrate, a dispersing agent and nitric acid, and a second aqueoussolution containing ascorbic acid.

The first aqueous solution in the present invention is an aqueoussolution containing silver nitrate, a dispersing agent and nitric acid.As the water used for the preparation of the first aqueous solution,pure water, ion-exchanged water, ultra-pure water or the like ispreferable for preventing impurities to be mixed. The silver nitrateused in the present invention is not specifically limited, but eithersolid or an aqueous solution can be used.

The examples of dispersing agents used in the present invention includepolyvinylpyrrolidone (PVP), gelatin, polyethylene glycol, polyvinylalcohol and the like. In the present invention, the term “gelatin” isused in the concept including glue. Among the dispersing agents used inthe present invention, polyvinylpyrrolidone and gelatin are preferablebecause the thermal shrinkage resistance of silver powder can beparticularly elevated. In the present invention, by compounding thedispersing agent in the first aqueous solution, the dispersion of thesilver powder can be improved, and there is the effect of making thesilver powder have fine particles, and making the particle sizedistribution not excessively sharp and relatively broad.

The nitric acid used in the present invention is not specificallylimited, but either concentrated nitric acid or diluted nitric acid canbe used. In the present invention, since the rate of the reaction toform silver from silver ions is controlled to be relatively slow bycompounding sulfuric acid in the first aqueous solution, there is theeffect of making the particle size distribution not excessively sharpand relatively broad, and enlarging the crystallites. If silver powderis produced without compounding nitric acid, the rate of the reaction toform silver from silver ions is excessively rapid and the reactionstarts immediately, the obtained silver powder has a smaller particlediameters, and the crystallite diameter tends to be reduced comparedwith the case to produce the silver powder by compounding nitric acid asin the present invention.

When the dispersing agent is polyvinylpyrrolidone, the first aqueoussolution contains normally 5 parts by weight to 60 parts by weight,preferably 15 parts by weight to 50 parts by weight, and more preferably20 parts by weight to 40 parts by weight of polyvinylpyrrolidone to 100parts by weight of silver nitrate. It is preferable that the compoundingquantity of polyvinylpyrrolidone is within the above-described range,because the dispersion of the silver powder is improved, and theparticle size distribution of the silver powder tends to be notexcessively sharp but to be relatively broad. On the other hand, it isnot preferable that the compounding quantity of polyvinylpyrrolidone isless than 5 parts by weight, because the obtained silver powder iseasily aggregated; and it is not preferable that the compoundingquantity of polyvinylpyrrolidone exceeds 60 parts by weight, because theimpurity concentration in the obtained silver powder is easily elevated,environment is easily contaminated, and the production costs tend to behigh.

When the dispersing agent is gelatin, the first aqueous solutioncontains normally 0.5 part by weight to 10 parts by weight, preferably 1part by weight to 8 parts by weight, and more preferably 2 parts byweight to 6 parts by weight of gelatin to 100 parts by weight of silvernitrate. It is preferable that the compounding quantity of gelatin iswithin the above-described range, because the dispersion of the silverpowder is improved, and the particle size distribution of the silverpowder tends to be not excessively sharp but to be relatively broad. Onthe other hand, it is not preferable that the compounding quantity ofgelatin is less than 0.5 parts by weight, because the obtained silverpowder is easily aggregated; and it is not preferable that thecompounding quantity of gelatin exceeds 10 parts by weight, because theimpurity concentration in the obtained silver powder is easily elevated,environment is easily contaminated, and the production costs tend to behigh.

When the dispersing agent is polyvinylpyrrolidone, the first aqueoussolution contains normally 1 part by weight to 10 parts by weight,preferably 2 parts by weight to 4 parts by weight of gelatin to 100parts by weight of water. It is preferable that the compounding quantityof polyvinylpyrrolidone is within the above-described range, because thedispersion of the silver powder is improved, and the particle sizedistribution of the silver powder tends to be not excessively sharp butto be relatively broad. On the other hand, it is not preferable that thecompounding quantity of polyvinylpyrrolidone is less than 1 parts byweight, because the obtained silver powder is easily aggregated; and itis not preferable that the compounding quantity of polyvinylpyrrolidoneexceeds 10 parts by weight, because the impurity concentration in theobtained silver powder is easily elevated, environment is easilycontaminated, and the production costs tend to be high.

When the dispersing agent is gelatin, the first aqueous solutioncontains normally 0.1 parts by weight to 5 parts by weight, preferably0.4 parts by weight to 2 parts by weight of gelatin to 100 parts byweight of water. It is preferable that the compounding quantity ofgelatin is within the above-described range, because the dispersion ofthe silver powder is improved, and the particle size distribution of thesilver powder tends to be not excessively sharp but to be relativelybroad. On the other hand, it is not preferable that the compoundingquantity of gelatin is less than 0.1 part by weight, because theobtained silver powder is easily aggregated; and it is not preferablethat the compounding quantity of gelatin exceeds 5 parts by weight,because the impurity concentration in the obtained silver powder iseasily elevated, environment is easily contaminated, and the productioncosts tend to be high.

The first aqueous solution contains normally 35 parts by weight to 70parts by weight, preferably 40 parts by weight to 60 parts by weight,and more preferably 48 parts by weight to 54 parts by weight of nitricacid to 100 parts by weight of silver nitrate. It is preferable that thecompounding quantity of nitric acid is within the above-described range,because the dispersion of the silver powder is improved, and theparticle size distribution of the silver powder tends to be notexcessively sharp but to be relatively broad. On the other hand, it isnot preferable that the compounding quantity of nitric acid is less than35 parts by weight, because the crystallization ability of the silverpowder is easily lowered; and it is not preferable that the compoundingquantity of nitric acid exceeds 70 parts by weight, because the obtainedsilver powder is easily aggregated. In the present invention, thecompounding quantity of nitric acid means the compounding quantityconverted to concentrate nitric acid of a concentration of 61%.

The second aqueous solution in the present invention is an aqueoussolution containing ascorbic acid. As the water used for the preparationof the first aqueous solution, pure water, ion-exchanged water,ultra-pure water or the like is preferable for preventing impurities tobe mixed. As the ascorbic acid used in the present invention, eitherL-isomer or D-isomer can be used.

In the production method according to the present invention, theabove-described first aqueous solution and second aqueous solution aremixed to deposit highly crystalline silver powder in the blendedsolution. The examples of mixing modes include a method wherein thefirst aqueous solution is agitated and the second aqueous solution ismixed thereto. As the method to add the second aqueous solution, theentire quantity of the second aqueous solution can be added to the firstaqueous solution at once, or the second aqueous solution can begradually added to the first aqueous solution a little at a time. Whenthe dispersing agent in the first aqueous solution ispolyvinylpyrrolidone, the method to add the entire quantity of thesecond aqueous solution to the first aqueous solution is preferablebecause the silver powder composed of fine particles, whose particlesize distribution is not excessively sharp but is relatively broad canbe easily obtained; when the dispersing agent in the first aqueoussolution is gelatin, the method to add the second aqueous solutiongradually to the first aqueous solution a little at a time is preferablebecause the particle diameter of silver powder can be easily controlled.

In mixing the first aqueous solution and the second aqueous solution,the solutions are mixed so that the quantity of ascorbic acid containedin the second aqueous solution is normally 30 parts by weight to 90parts by weight, preferably 40 parts by weight to 80 parts by weight,and more preferably 50 parts by weight to 75 parts by weight to 100parts by weight of silver nitrate contained in the first aqueoussolution. It is preferable that the compounding quantity of ascorbicacid to silver nitrate is within the above-described range because theyield of the silver powder is easily elevated. On the other hand, it isnot preferable that the compounding quantity of ascorbic acid to 100parts by weight silver nitrate is less than 30 parts by weight, becausereduction is insufficient and the yield of the silver powder is easilylowered; and it is not preferable that the compounding quantity ofascorbic acid to 100 parts by weight silver nitrate exceeds 90 parts byweight, because environment is easily contaminated, and the productioncosts tend to be high.

In mixing the first aqueous solution and the second aqueous solution,the solutions are mixed so that the silver ion concentration in theobtained mixed solution is normally 10 g/l to 80 g/l, preferably 30 g/lto 65 g/l. It is preferable that the silver ion concentration in theblended solution is within the above-described range because the yieldof the silver powder is high and the obtained silver powder is difficultto aggregate. On the other hand, it is not preferable that the silverion concentration is less than 10 g/l, the productivity of the silverpowder tends to be worsened; and it is not preferable that the silverion concentration exceeds 80 g/l, the silver powder is easilyaggregated.

In mixing the first aqueous solution and the second aqueous solution,the solutions are mixed so that the quantity of nitric acid contained inthe first aqueous solution is normally 40 parts by weight to 150 partsby weight, preferably 50 parts by weight to 120 parts by weight, andmore preferably 65 parts by weight to 100 parts by weight to 100 partsby weight of ascorbic acid contained in the second aqueous solution. Itis preferable that the compounding quantity of nitric acid to ascorbicacid is within the above-described range because the yield of the silverpowder is easily elevated. On the other hand, it is not preferable thatthe compounding quantity of nitric acid to 100 parts by weight ascorbicacid is less than 40 parts by weight, because it is difficult tosufficiently increase the crystallite diameter of the obtained silverpowder; and it is not preferable that the compounding quantity of nitricacid to 100 parts by weight ascorbic acid exceeds 150 parts by weight,the obtained silver powder is easily aggregated.

It is preferable that silver powder deposited in the blended solution bymixing the first aqueous solution and the second aqueous solution isgrown in the blended solution by continuing agitation normally forfurther 3 minutes or more, preferably 5 minutes to 10 minutes, becausethe particle diameter, particle size distribution and dispersion of thesilver powder are easily within the specific ranges of the silver powderaccording to the present invention. After filtering the silver powderobtained in the blended solution using filtering means, for example, aNutsche, the filtered product is washed with pure water and dried toobtain the highly crystalline silver powder according to the presentinvention.

The above-described highly crystalline silver powder according to thepresent invention can be used as the material for a conductive pastethat can form electrodes or circuits of, for example, chip devices,plasma display panels, glass ceramic packages, ceramic filters and thelike; in particular, it can be suitably used as the material for aconductive paste not only for ordinary ceramic substrates, but also forLTCC substrates as substrates forming the circuit, utilizing the verysmall thermal shrinkage of the silver powder. The method for producinghighly crystalline silver powder according to the present invention canalso be used for producing highly crystalline silver powder according tothe present invention.

Although the examples will be described below, these examples should notbe construed to limit the present invention.

EXAMPLE 1

10 g of PVP (K-value: 30), 50 g of silver nitrate and 24.6 g ofconcentrated nitric acid (concentration: 61 wt %) were added into 500 gof pure water at room temperature, and dissolved by stirring to preparea first aqueous solution (first aqueous solution A). In addition, 35.8 gof ascorbic acid was added into 500 g of pure water at room temperature,and dissolved by stirring to prepare a second aqueous solution (secondaqueous solution A). The compositions of the first aqueous solution andthe second aqueous solution are shown in Table 1 and Table 2.

Next, the second aqueous solution A was added to the stirring firstaqueous solution A at once, after that, stirring was continued for 5minutes to grow particles in the blended solution. Thereafter, stirringwas stopped to settle the particles in the blended solution. Aftersettling the particles, the supernatant of the blended solution wasdisposed, and the rest of blended solution was filtered using a Nutsche.Then the filtered product was rinsed with pure water and dried, and thenhighly crystalline silver powder was obtained.

For the obtained silver powder, D₁₀, D₅₀, D₉₀, D₁₀₀, SD, crystallitediameter, specific surface area, tap density, thermal shrinkage andresistivity were measured using the following methods and D₉₀/D₁₀ wascalculated. The results are shown in Table 3 to Table 6.

(D₁₀, D₅₀, D₉₀, D₁₀₀, SD): 10%, 50%, 90% and 100% were indicated as D₁₀(μm), D₅₀ (μm), D₉₀ (μm), D₁₀₀ (μm) are particle diameters when thecumulative distributions measured by a laser diffraction scatteringmethod using [Micro Track HRA] manufactured by Nikkiso Co., Ltd. were,respectively, and SD is the standard deviation in the particle sizedistribution.(Crystallite diameter): X-ray diffraction on the powder was performedusing an X-ray diffraction apparatus [RINT 2000/PC] manufactured byRigaku Corporation, and the crystallite diameter was calculated from thehalf-value width of the peak of diffraction angle obtained on eachcrystal faces.(Specific surface area): It is a B.E.T. specific surface area measuredby [Monosorb] manufactured by Yuasa-Ionics Co., Ltd.(Tap density): Tap density was measured by tapping the sample using [TapDenser] manufactured by Kuramochi Kagaku Kikai Seisakusyo Co., Ltd.(Thermal shrinkage): A columnar pellet was prepared by compressing thesilver powder, and the TMA analysis of the pellet was conducted usingTMA/SS 6300 manufactured by Seiko Instruments Inc., in air at atemperature elevation rate of 10° C./min within the range between roomtemperature and 850° C. to measure thermal shrinkage of the pellet inthe length direction. The measuring temperatures were 300° C., 500° C.and 700° C.(Resistivity): A mixed solvent was prepared by mixing 95 parts by weightof terpineol and 5 parts by weight of ethyl cellulose, a paste wasprepared by mixing 15 parts by weight of the mixed solvent and 85 partsby weight of the sample powder, and the paste was baked at 300° C. toprepare a silver coating film having a thickness of about several μm.Other silver coating films were prepared in the same manner as describedabove except that the baking temperatures were 400° C. and 500° C.instead of 300° C.

Then, after measuring the resistance (Ω) of the silver coating films byfour-terminal network method using (MILLIOHM METER manufactured byHewlett-Packard), the resistivity ρ (Ω·m) was obtained from thecross-sectional area of the silver coating films and the length betweenthe terminals.

EXAMPLE 2

20 g of PVP (K-value: 30), 50 g of silver nitrate and 24.6 g ofconcentrated nitric acid (concentration: 61%) were added into 500 g ofpure water at room temperature, and dissolved by stirring to prepare afirst aqueous solution (first aqueous solution B). In addition, 35.8 gof ascorbic acid was added into 500 g of pure water at room temperature,and dissolved by stirring to prepare a second aqueous solution (secondaqueous solution A). The compositions of the first aqueous solution andthe second aqueous solution are shown in Table 1 and Table 2.

Next, the second aqueous solution A was added to the first aqueoussolution B in at once, after that, stirring was continued for 5 minutesto grow particles in the blended solution. Thereafter, stirring wasstopped to settle the particles in the blended solution. After settlingthe particles, the supernatant of the blended solution was filteredusing a Nutsche. Then, the filtered product was rinsed with pure waterand dried, and then highly crystalline silver powder was obtained.

For the obtained silver powder, D₁₀, D₅₀, D₉₀, D₁₀₀, SD, crystallitediameter, specific surface area, tap density, thermal shrinkage andresistivity were measured in the same manner as in Example 1 using thefollowing methods and D₉₀/D₁₀ was calculated. The results are shown inTable 3 to Table 6.

COMPARATIVE EXAMPLE 1

10 g of PVP (K-value: 30) and 50 g of silver nitrate were added into 500g of pure water at room temperature, and dissolved by stirring toprepare a first aqueous solution (first aqueous solution C). Inaddition, 26 g of ascorbic acid was added into 500 g of pure water atroom temperature, and dissolved by stirring to prepare a second aqueoussolution (second aqueous solution B). The compositions of the firstaqueous solution and the second aqueous solution are shown in Table 1and Table 2.

Next, the second aqueous solution B was added to the stirring firstaqueous solution C at once, after that, stirring was continued for 5minutes to grow particles in the blended solution. Thereafter, stirringwas stopped to settle the particles in the blended solution. Aftersettling the particles, the supernatant of the blended solution wasdisposed, and the rest of blended solution was filtered using a Nutsche.Then the filtered product was rinsed with pure water and dried, and thenhighly crystalline silver powder was obtained.

For the obtained silver powder, D₁₀, D₅₀, D₉₀, D₁₀₀, SD, crystallitediameter, specific surface area, tap density, thermal shrinkage andresistivity were measured in the same manner as in Example 1 using thefollowing methods and D₉₀/D₁₀ was calculated. The results are shown inTable 3 to Table 6.

EXAMPLE 3

1.0 g of gelatin (manufactured by Nitta Gelatin Inc.), 50 g of silvernitrate and 24.6 g of concentrated nitric acid (concentration: 61%) wereadded into 250 g of pure water at room temperature, and then, heated upto 50° C. and dissolved by stirring to prepare a first aqueous solution(first aqueous solution D). In addition, 26.4 g of ascorbic acid wasadded into 250 g of pure water at room temperature, and dissolved bystirring to prepare a second aqueous solution (second aqueous solutionC). The compositions of the first aqueous solution D and the secondaqueous solution C are shown in Table 1 and Table 2.

Next, the second aqueous solution C at room temperature was graduallyadded to the stirring first aqueous solution D at 50° C. in 30 minutes,after that, stirring was continued for 5 minutes to grow particles inthe blended solution. Thereafter, stirring was stopped to settle theparticles in the blended solution. After settling the particles, thesupernatant of the blended solution was disposed, and the rest ofblended solution was filtered using a Nutsche. Then the filtered productwas rinsed with pure water and dried, and then highly crystalline silverpowder was obtained.

For the obtained silver powder, D₁₀, D₅₀, D₉₀, D₁₀₀, SD, crystallitediameter, specific surface area, tap density, thermal shrinkage andresistivity were measured in the same manner as in Example 1 using thefollowing methods and D₉₀/D₁₀ was calculated. The results are shown inTable 3 to Table 6.

EXAMPLE 4

3.0 g of gelatin (manufactured by Nitta Gelatin Inc.), 50 g of silvernitrate and 24.6 g of concentrated nitric acid (concentration: 61%) wereadded into 500 g of pure water at room temperature, and then, heated upto 50° C. and dissolved by stirring to prepare a first aqueous solution(first aqueous solution E). In addition, 25.9 g of ascorbic acid wasadded into 500 g of pure water at room temperature, and dissolved bystirring to prepare a second aqueous solution (second aqueous solutionD). The compositions of the first aqueous solution and the secondaqueous solution are shown in Table 1 and Table 2.

Next, the second aqueous solution D at room temperature was graduallyadded to the stirring first aqueous solution E in 30 minutes, afterthat, stirring was continued for 5 minutes to grow particles in theblended solution. Thereafter, stirring was stopped to settle theparticles in the blended solution. After settling the particles, thesupernatant of the blended solution was disposed, and the rest ofblended solution was filtered using a Nutsche. Then the filtered productwas rinsed with pure water and dried, and then highly crystalline silverpowder was obtained.

For the obtained silver powder, D₁₀, D₅₀, D₉₀, D₁₀₀, SD, crystallitediameter, specific surface area, tap density, thermal shrinkage andresistivity were measured in the same manner as in Example 1 using thefollowing methods and D₉₀/D₁₀ was calculated. The results are shown inTable 3 to Table 6.

TABLE 1 Kind of Kind Dispersing Silver Concentrated first of dispersingagent nitrate nitric acid aqueous Water (g) agent (g) (g) (g) solutionsExample 1 500 PVP 10 50 24.6 A Example 2 500 PVP 20 50 24.6 BComparative 500 PVP 10 50 0 C Example 1 Example 3 250 Gelatin 1.0 5026.4 D Example 4 500 Gelatin 3.0 50 24.6 E

TABLE 2 Kind of Ascorbic second Water acid aqueous (g) (g) solutionsExample 1 500 35.8 A Example 2 500 35.8 A Comparative 500 26.0 B Example1 Example 3 250 26.4 C Example 4 500 25.9 D

TABLE 3 D₁₀₀ D₁₀ (μm) D₅₀ (μm) D₉₀ (μm) (μm) D₉₀/D₁₀ SD Example 1 2.976.35 10.75 22.0 3.6 3.01 Example 2 1.30 3.03 5.67 15.6 4.4 1.59Comparative 2.14 2.83 4.08 9.3 1.9 0.71 Example 1 Example 3 2.72 4.367.33 18.5 2.7 1.71 Example 4 0.76 1.27 2.28 4.6 3.0 0.57

TABLE 4 Crystallite Specific diameter surface area Tap density (Å)(m²/g) (g/cm³) Example 1 441 0.30 4.1 Example 2 377 0.62 4.0 Comparative258 0.62 3.8 Example 1 Example 3 545 0.20 4.4 Example 4 441 0.72 4.8

TABLE 5 Thermal Thermal Thermal shrinkage shrinkage shrinkage percentageat percentage at percentage at 300° C. 500° C. 700° C. (%) (%) (%)Example 1 0.13 −2.13 −2.2 Example 2 0.09 −2.68 −2.9 Comparative 0.84−4.02 −7.82 Example 1 Example 3 0.27 1.08 1.13 Example 4 −0.58 −1.51−1.35

TABLE 6 Resistivity of Resistivity of Resistivity of silver coatingsilver coating silver coating film baked at film baked at film baked at300° C. ρ(Ω · m) 400° C. ρ(Ω · m) 500° C. ρ(Ω · m) Example 1 4.1 × 10⁻⁵2.0 × 10⁻⁵ 9.9 × 10⁻⁶ Example 2 5.2 × 10⁻⁵ 1.5 × 10⁻⁵ 1.2 × 10⁻⁵Comparative 7.2 × 10⁻⁴ 8.9 × 10⁻⁶ 4.8 × 10⁻⁵ Example 1 Example 3 9.4 ×10⁻⁶ 8.3 × 10⁻⁶ 9.9 × 10⁻⁶ Example 4 1.0 × 10⁻⁵ 8.8 × 10⁻⁶ 4.8 × 10⁻⁵

From Table 1 to Table 5, it is clearly shown that silver powder preparedby using both dispersing agent and nitric acid is highly crystallinewith a large crystallite diameter, and thermal shrinkage after heatingat 700° C. is small. Especially when gelatin is used as the dispersingagent, thermal shrinkage after heating at 700° C. is especially small.From Table 6, it is found that silver powder prepared by using bothdispersing agent and nitric acid show lower resistivity ρ on the silvercoating film after baked at 300° C. when compared with the silver powderprepared without using nitric acid. The reason why is suspected that themovement of electrons in the silver powder of the invention is moresmooth because of it is large crystallite diameter.

INDUSTRIAL APPLICABILITY

The highly crystalline silver powder according to the present inventionis useful for the material to be contained in a conductive paste forforming electrodes and/or circuits, for example, chip devices, plasmadisplay panels, glass ceramic packages, ceramic filters and the like.Also the method for producing highly crystalline silver powder accordingto the present invention is useful. Especially, it is shows goodperformance on LTCC substrate.

1. A method for producing highly crystalline silver powder which ischaracterized in that mixing a first aqueous solution and a secondaqueous solution, wherein the first aqueous solution contains silvernitrate, a dispersing agent and nitric acid, and the second solutioncontains ascorbic acid.
 2. The method for producing highly crystallinesilver powder according to claim 1, wherein the dispersing agent ispolyvinylpyrrolidone.
 3. The method for producing highly crystallinesilver powder according to claim 1, wherein the dispersing agent is agelatin.
 4. The method for producing highly crystalline silver powderaccording to claim 2, the first aqueous solution when it contains 100parts by weight of silver nitrate, it further contains 5 parts by weightto 60 parts by weight of polyvinylpyrrolidone and 35 parts by weight to70 parts by weight of nitric acid.
 5. The method for producing highlycrystalline silver powder according to claim 3, the first aqueoussolution when it contains 100 parts by weight of silver nitrate, itfurther contains 0.5 parts by weight to 10 parts by weight of gelatinand 35 parts by weight to 70 parts by weight of nitric acid.
 6. Themethod for producing highly crystalline silver powder according to claim1, when the first aqueous solution contains 100 parts by weight ofsilver nitrate, ascorbic acid contained in the second aqueous solutionto be mixed with the first aqueous solution is 30 parts by weight to 90parts by weight.
 7. The method for producing highly crystalline silverpowder according to claim 1, when the second aqueous solution contains100 parts by weight of ascorbic acid, nitric acid contained in the firstaqueous solution to be mixed with the second aqueous solution is 40parts by weight to 150 parts by weight.
 8. Highly crystalline silverpowder which is characterized in that the powder is produced by themethod according to claim
 1. 9. The highly crystalline silver powderaccording to claim 8, wherein crystallite diameter of the powder is 300Å or more.
 10. The highly crystalline silver powder according to claim8, wherein an average particle diameter D₅₀ of the powder is in therange from 0.5 μm to 10 μm. (where D₅₀ is a median diameter (μm)calculated as 50% of volume cumulative distributions examined by a laserdiffraction scattering particle size distribution measuring method). 11.The highly crystalline silver powder according to claim 8, wherein athermal shrinkage rate of the powder after heating at 700° C. is in therange from −3% to 3%.
 12. The highly crystalline silver powder accordingto claim 8, wherein a ratio D₉₀/D₁₀ of the powder is in the range from2.1 to 5.0 (where D₁₀ is diameter (μm) at 10% of volume cumulativedistributions and D₉₀ is diameter (μm) at 90% of volume cumulativedistributions examined by a laser diffraction scattering particle sizedistribution measuring method, respectively).
 13. Highly crystallinesilver powder which is characterized in that a crystallite diameter is300 Å or more, an average particle diameter D₅₀ is in the range from 0.5μm to 10 μm, and a thermal shrinkage ratio after heating at 700° C. inthe length direction is in the range from −3% to 3%.
 14. The highlycrystalline silver powder according to claim 13, wherein a ratio D₉₀/D₁₀of the powder is in the range from 2.1 to 5.0 (where D₁₀ is diameter(μm) at 10% of volume cumulative distributions and D₉₀ is diameter (μm)at 90% by volume of cumulative distributions examined by a laserdiffraction scattering particle size distribution measuring method,respectively).